FIELD OF THE INVENTION
The invention relates, in general, to cryogenic systems that include an apparatus configured to deliver current to a cooled electrical device, and in particular, to an apparatus for delivering current to a cooled electrical device. The apparatus has at least one electrical line that runs between a location at a higher temperature level and a location at a respectively lower temperature level. The electrical line has an end at the low-temperature location that is connected to a cooled electrical device. Such an apparatus for delivering current has been described, for example, in the journal "Cryogenics", Vol. 25, 1985, pages 94 to 110.
One of the main problems in the design of cryogenic systems is the efficient introduction of relatively large currents into superconducting or semiconductive devices. Examples of such superconducting or semiconductive devices include those that are used to produce magnetic fields, to provide short-circuit current limiting, to transform voltages, and to transmit power. The greatest source of heat leakage in an insulated cryogenic container is often an electrical conductor of a power supply. The conductor runs between a location at a high temperature level, in particular room temperature at about 300 K, and a location at a lower temperature level of, for example, 77 K which is the temperature of liquid nitrogen LN.sub.2. An electrical device such as a superconducting or semiconducting device is at the low temperature location and is preferably maintained at the low temperature. If the electrical conductor or line that runs between the locations of these temperature levels cannot be designed to have low losses, and if the heat losses that are incurred are not effectively dissipated, the cooling cost may call into question the technical or economic purpose of the entire system.
In regard to the design of known apparatus for delivering current to a cooled electrical device, a distinction is drawn between line-cooled and exhaust-gas-cooled types. Line-cooled apparatus for delivering current are in general cooled only by heat conduction from a cold end. The dimensions of the line can be optimized so that the sum of the I.sup.2 R-losses from the metal in a line having a specific resistance .rho. (T) and the transported heat governed by the temperature-dependent thermal conductivity .lambda. (T) is a minimum. If the dimensions are optimized, then the specific loss--the heat introduced per unit of current, is about 43 W/kA for copper, based on a single electrical line (see the journal "IEEE Transactions on Magnetics", Vol. MAG-13, No. 1, 1977, pages 690 to 693).
With exhaust-gas-cooled apparatus for delivering current, the enthalpy of a vaporized coolant, for example liquid nitrogen LN.sub.2 at a temperature of 77 K or liquid helium LHe at a temperature of 4.2 K, is used to dissipate the introduced heat losses in the opposite direction. The specific loss between 300 K and 77 K can be reduced to about 25 W/kA, with about 0.56 liters of LN.sub.2 being vaporized per hour, per kiloampere, and per power supply line.
In a line-cooled apparatus for delivering current, the amount of heat introduced into a cryostat dictates the operating period of the cryogenic system before replenishment of the coolant supply is required. In an exhaust-gas-cooled apparatus for delivering current, the amount of heat introduced into a cryostat dictates the size of the required cooling unit.
The amount of power that must be provided for cooling at room temperature is also important for a user. This power is consumed, for example, in a compressor of a cooling unit or in the production of the liquid coolant.
A number of embodiments of apparatus for delivering current to a cooled electrical device are known. A particular embodiment is selected depending upon the specific application (See the journals referenced above). As a rule, the material used for the electrical line that runs between the locations at different temperature levels is copper or brass. In the case of line-cooled apparatus for delivering current, the cold end is also frequently highly thermally conductive, but is connected in an electrically insulated manner to the cold side of a refrigerator that operates in accordance with the Gifford-McMahon principle. In the case of exhaust-gas-cooled apparatus for delivering current, at least a large proportion of the vaporized coolant is passed along the electrical line. The surface area of the electrical line should be as large as possible so that effective heat exchange takes place.